1. Field of the Invention
The present invention relates to a color video camera capable of providing good color reproducibility even under illumination with discharge lamps such as white fluorescent lamps having an inferior color rendering property.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a prior art color video camera disclosed in Japanese Utility Model Publication No. 41-16011. Referring to the diagram, reference numeral 1 denotes a lens, 2 denotes an image pickup device, 3 denotes a color separator, 4 denotes an R-channel gain control circuit, 5 denotes a B-channel gain control circuit, 6 denotes a processor circuit, 7 denotes a matrix circuit, 8 denotes an encoder, 9 denotes a sync signal generator, 10 denotes an output terminal, 21 denotes an R sensor (photoelectric transfer device), 22 denotes a G sensor (photoelectric transfer device), 23 denotes a B sensor (photoelectric transfer device), 24 and 25 denote dividers, 28 and 29 denote linear converter circuits, 30 denotes an fscR-Y phase control circuit for varying the phase of a chrominance subcarrier fscR-Y modulated by a color difference signal R-Y, 31 denotes an fscB-Y phase control circuit for varying the phase of a chrominance subcarrier fscB-Y modulated by a color difference signal B-Y, and 40 denotes a color reproducibility compensation circuit formed of the phase control circuits 30 and 31.
Operation of the camera will be described below. An optical image incoming through the lens 1 is focused onto and photoelectrically transformed by the image pickup device 2 and separated into three color signals R, G, and B by the color separator 3. Then, the R-channel gain control circuit 4 controls the gain of the R signal in the R channel and the B-channel gain control circuit 5 controls the gain of the B signal in the B channel, whereby adjustment of white balance is performed.
On the other hand, the R sensor 21, G sensor 22, and B sensor 23 generate outputs respectively proportional to the R component, G component, and B component of the incident light. Signals R.sub.S, G.sub.S and B.sub.S respectively representing values of these outputs, the divider 24 outputs a ratio of the G component to the R component, G.sub.S /R.sub.S (=V.sub.R), which is not dependent on quantity of the incident light. Likewise, the divider 25 outputs a ratio of the B component to the G component, B.sub.S /G.sub.S (=V.sub.B) Here, the gain control circuit 4 is adapted such that its gain becomes greater as the control voltage V.sub.R becomes greater, whereas the other gain control circuit 5 is adapted such that its gain becomes smaller as the control voltage V.sub.B becomes greater. Accordingly, in the case where a color source is of a low color temperature, i.e., where the R component is greater and the B component is smaller as against the G component, the ratios V.sub.R =G.sub.S /R.sub.S and V.sub.B =B.sub.S /G.sub.S both become smaller in value than those in the case of a light source of a high color temperature and, hence, the gain of the R-channel gain control circuit 4 becomes smaller, while the gain of the B-channel gain control circuit 5 becomes greater.
On the other hand, in the case where a color source is of a high color temperature, i.e., where the R component is smaller and the B component is greater as against the G component, the gain of the R-channel gain control circuit 4 becomes greater while the gain of the B-channel gain control circuit 5 becomes smaller. In the described manner, the adjustment for white balance is performed through automatic control of the gain of the R-channel gain control circuit 4 and the gain of the B-channel gain control circuit 5 in response to the changes in the R component, B component, and G component of the incident light source. When the color temperature varies due to such factors as the spectral sensitivity characteristic of the image pickup device 2 and the signal processing method, the phase of the color signal varies even if the white balance is adjusted correctly. Therefore, the values V.sub.R and V.sub.B are subjected to a linear conversion in the linear converter circuits 28 and 29, respectively, and the outputs are supplied to the phase control circuits 30 and 31, and thereby, compensation for changes in the phase of the color signal is performed.
FIG. 2 is a vector diagram indicating a case where red, yellow, skin color, and white charts are picked up in fine afternoon weather (before a sunset, at a color temperature around 4000.degree. K.) by the color video camera as described above, and FIG. 3 is a vector diagram in the case where red, yellow, skin color, and white charts are picked up under white fluorescent lamp illumination (at a color temperature around 4000.degree. K.) by the same. In either case, the voltage values controlling the gains of the gain control circuits 4 and 5 are virtually equal and the adjustments for white balance are correctly performed.
Further, there is known another prior art circuit in which, instead of the phase control circuits 30 and 31 in FIG. 1, gain control circuits are inserted in the lines for color difference signals R-Y and B-Y between the matrix circuit 7 and the encoder 8 so that the color difference signals R-Y and B-Y are corrected for changes in amplitude produced at the time an adjustment is made for white balance.
Furthermore, there is known another prior art example in which, instead of the phase control circuits 30 and 31 in FIG. 1, mixer circuits are inserted in the lines for color difference signals R-Y and B-Y between the matrix circuit 7 and the encoder 8 so that mixing rates of the color difference signals R-Y and B-Y are controlled by the outputs of the linear converter circuits 28 and 29 and thereby the changes in the phase are corrected.
With prior video cameras arranged as described above, phase and amplitude of red, yellow, and skin color generally become worse under white fluorescent lamp illumination as compared to afternoon sunlight illumination as will be known by comparing FIG. 3 with FIG. 2, because of the inferior color rendering property of the white fluorescent lamp illumination. As to the phase of skin color, in particular, though it is at a preferable phase of 123.degree. in FIG. 2, the phase is shifted to 137.degree. in FIG. 3, that is, the skin color becomes yellowish, and this poses a problem in visual sensitivity. Further, since the skin color is a memory color, it is desirable that it have a constant phase and amplitude regardless of the kind of light source. However, the allowance is especially narrow for the change in its phase and, hence, there has been a problem that the variations in the phase of the skin color become marked particularly under white fluorescent lamp illumination having such inferior color rendering property. A method for improving color reproducibility with attention only paid to change in color temperature is described, for example, in a paper, "Improvement in Color Reproducibility in a Camera of Full Color Difference Line Sequential System", Preliminary Papers for Nationwide University Lectures of Japan Society of Television Engineers, pp. 81-82, 1986, but sufficient effects have not been obtained as yet by these methods under a light source with such an inferior color rendering property as described above.
In the above described type provided with gain control circuits instead of the phase control circuits 30 and 31, such a prior art color video camera was adapted to control the gains of the color difference signals R-Y and B-Y with values obtained by a linear conversion of the values V.sub.R (=G.sub.S /R.sub.S) and V.sub.B (=G.sub.S /B.sub.S). Hence, although it is desirable that the skin color, being a memory color, is reproduced with a constant phase and amplitude regardless of the kind of light sources, there was a problem that under a light source having an inferior color rendering property such as white fluorescent lamp illumination, phase and amplitude under such white fluorescent lamp illumination are shifted as shown in FIG. 5 from the case in fine afternoon weather as shown in FIG. 4.